1. Field of the Invention
This invention is generally directed to nails and other types of fasteners and more specifically to sheet metal fasteners of the type which are utilized to secure membranes to cementitious material such as utilized in roofing applications wherein the fasteners are designed to have a shank or shanks having leg portions which when driven through the membrane and into the cementitious material will expand to retain a plug of the cementitious material to thereby prevent the fasteners from being removed from the cementitious material. Even more specifically, the present invention is directed to such fasteners that are designed to be corrosion resistant and wherein the head portion of the fasteners and any washers or discs that may be utilized in conjunction therewith are coated with corrosion resistant materials leaving at least a substantial portion of the shank or legs of the fasteners uncoated so that the fasteners may chemically react with the cementitious material to establish a strong chemical bond therebetween.
2. History of the Related Art
Lightweight cementitious roofing systems have long been utilized in the construction industry. Such systems employ a slurry of Portland cement and water which is impregnated with either a foam or aggregate material. The uncured cementitious material is generally poured over metal decking or concrete decking which forms the reinforcing components of the roofing structure. In some such roofing systems an insulating material is placed atop the roofing substructure and thereafter encapsulated into a poured cementitious material such as a lightweight insulating concrete. Once the roof deck is poured, it is covered by a roofing membrane which is frequently a base ply which is fastened to the cementitious material by the use of fasteners of the type disclosed in U.S. Pat. No. 3,710,627 to Robert L. Hallock. These fasteners include shanks formed of opposing legs which are designed to spread relative to one another so as to enclose a plug of cementitious material therebetween when the fasteners are driven through the base ply and into the cementitious material. Such fasteners are designed to withstand removal by securing a plug of material which is of a size greater than the opening into or through the base ply. In some instances a separate washer or disc is utilized in conjunction with the fasteners. The disc, like the fastener, may be galvanized. The discs are designed to distribute stresses so as to prevent rupturing of the membrane or base ply covering the cementitious material. Examples of such discs are disclosed in U.S. Pat. No. 4,627,207 to Young et al.
After the initial layer of membrane has been secured by mechanical fasteners to the cementitious material, additional plies of roofing membrane may be applied over the fasteners and membrane using conventional sealing agents. Such an assembly is a conventional "built-up roof". Other water proofing membranes may be applied either directly atop the cementitious roof deck or to the fastened base ply.
To ensure the integrity of such cementitious roofing systems and other building products, parameters have been set forth by testing agencies such as Underwriters Laboratories, Inc. and Factory Mutual Research Corporation. One such parameter or guideline established by an independent testing facility to determine a product's resistance to corrosive environments has been referred to as a "Kesternich test". The test is used to determine resistance to corrosion of mechanical fasteners used in roofing systems. The test sets forth specific requirements that such fasteners and the washers or discs utilized in conjunction therewith exhibit a resistance to the formation of corrosion. It has been noted over a period of years that sheet metal fasteners corrode at a rate in excess of that which a testing agency has determined acceptable for the long term integrity of roofing systems.
It has been recognized that galvanized coatings associated with conventional metallic fasteners develop a bond with cementitious roof decking thereby increasing the withdrawal resistance of the fasteners over a period of time. As much as 50% of the long-term withdrawal resistance of the fasteners is attributable to the chemical reaction between the galvanized coating on the fasteners and the cementitious material in a roof deck. Therefore, any attempt to alter the surface or chemical characteristics of fasteners and especially galvanized fasteners would result in a significant reduction of the withdrawal resistance of such fasteners.
An independent testing agency has determined that under certain conditions corrosion may develop along the head portion of such conventional fasteners after installation. To overcome the problems of corrosion it was originally believed that such fasteners could simply be completely coated with a corrosion resistant paint or other material and thereafter applied in a conventional manner to a roof deck system. However, through testing it has become apparent that coating all portions of such fasteners negates the bond that would otherwise develop between the fasteners and the cementitious material. A chemical reaction occurs between the zinc of the galvanized coating of the sheet metal of the fasteners and the cementitious material which results in the formation of a strong bond. As previously discussed, it has been determined that this bond may account for as much as 50% of the long-term withdrawal resistance of the fasteners and is therefore essential to the long-term performance of such fasteners and roof deck systems utilizing such fasteners.
Some additional examples of conventional roof deck fasteners include U.S. Pat. Nos. 3,466,967, 3,878,756 and 4,213,373 to Hallock. Some additional examples of discs or washers utilized with mechanical fastening systems in cementitious roof deck systems are disclosed in U.S. Pat. Nos. 4,641,471 and 4,641,472 to Young et al.